Maxwell's Demon
A tiny creature sorting fast molecules from slow ones appears to make heat flow the wrong way, for free, forever.
1867 (letter to Tait), 1871 (published in Theory of Heat)18 min read
Scottish mathematical physics
James Clerk Maxwell
1831 – 1879
Author of the equations that unified electricity, magnetism and light, and a founder of the statistical theory of gases. In an 1867 letter to Peter Guthrie Tait he imagined "a very observant and neat-fingered being" who could sort fast molecules from slow ones and so, he said, "pick a hole" in the second law of thermodynamics. He published the thought experiment in Theory of Heat (1871). The word "demon" was William Thomson's, not his, and Maxwell disliked it.